The ability to measure the voltage carried by an asset, e.g., a conductor, can be especially important to electric utility companies. Conventionally, voltage measurement of high voltage assets up to 765 kV was accomplished through the use of potential transformers (“PTs”) and capacitively coupled voltage transformers (“CCVTs”). As shown in FIG. 1, a CCVT comprises a capacitor divider circuit, and the voltage induced on both the capacitors is proportional to the asset voltage. Thus, the asset voltage can be determined by measuring the voltage across one of the capacitors. However, as this technique requires two physically connected capacitors across a high voltage asset and ground, it has stringent insulation requirements. This requirement increases the design challenges, size, and cost of these sensors. Accordingly, it is presently not possible to use this technology for voltage sensing in low-cost sensors.
To reduce the insulation requirements of the voltage sensor, the sensor can be floated at the same potential as the asset, as shown in FIGS. 2A-2B. In this case, the air between the sensing plate S1 and the ground acts as a dielectric medium between capacitance to ground. The capacitor C1 is then used to measure the voltage of the conductor. A low impedance integrating amplifier between the sensing plate and the conductor can be added which brings the sensing plate to the asset potential and effectively eliminates C1 from the circuit. The displacement current in capacitor C2 flows through CF of the op-amp and results in a voltage output across the op-amp which is directly proportional to the asset voltage. A major drawback that ensues with this approach pertains to the deposition of water drops or snow on the sensing plate, changing the displacement current flowing through the op-amp. To minimize the errors due to this effect, the width and length of the sensing plate has to be very large as compared to the gap between the two sensing plates. This approach, however, is not immune to the effect of tree branches in the vicinity of the asset or the presence of multiple assets in the vicinity. Moreover, the physical geometry requirements to reduce spurious external effects are demanding and tend to increase the overall size of the sensor.
To tackle the effects of vegetation, distance to ground, and nearby assets, another conventional sensor uses a circular array of capacitor plates as shown in FIG. 3. The primary idea behind this technique is the use of multiple capacitors (six capacitor plates) for eliminating the effects due to external conditions in voltage measurements. The displacement current flowing through each of the capacitor plates has information embedded in it related to external conditions, such as geometry of nearby conductors and vegetation. This approach, however, suffers from a major drawback in that it requires six capacitor plates distributed in space encircling the conductor which increases the size of the sensor. Moreover, this approach can only be used with conductors and does not have the flexibility of being used in conjunction with other assets. Furthermore the algorithms are fairly involved and require increased computation power to solve for the voltage, phase angle, and conductor clearance. This increased computation demands more power for operating the sensor, and is a cause for concern in a self-powered low-cost sensing application. In addition, the algorithms are based on the premise that the conductor is part of a three-phase system and cannot operate in a single-phase electrical system.
These conventional sensors make the use of a floating sensor on a high voltage asset look promising as they are free from high voltage insulation requirements. These sensors, however suffer from at least the following limitations: most of the sensors require field calibration that is very expensive; the construction of the sensor is challenging; the algorithms used to compute the voltage is complex and requires a lot of computing power; the implementation of the sensor in a low-power module is difficult; the sensors are constrained in their application (limited to three-phase systems); and the sensors are sensitive to variations in distance to ground, nearby assets, electric fields from nearby assets, and changes in atmospheric conditions. Additionally, these conventional solutions aim at developing voltage sensors for revenue grade metering applications. In such applications, the errors are required to be on the orders of 0.1-1%. Therefore, there exists a gap in the opportunity space for applications that require moderate accuracy of voltage sensing.
Therefore, there is a desire for voltage sensor systems and methods that address one or more of the disadvantages discussed above. Various embodiments of the present invention address these desires.